1. Field of the Invention
The present invention relates to the field of sensing equipment and, more particularly, to a technique of measuring RF voltage and current.
2. This application is related to patent application Ser. No. 08/742,393; filed Oct. 31, 1996, now pending; and titled "RF Current Sensor."
3. Background of the Related Art
In the area of radio frequency (RF) signal monitoring, the art abounds with various devices and techniques for measuring an RF signal. However, for a sensor to have consistent accuracy that can be relied upon without resorting to systematic calibration of the sensor, the sensor should operate upon characteristics defined by first principles. The first principle standards operate on properties that are calculated from elemental physical characteristics. One such example is the zero-state transition of Cesium, which is used as a time standard. In RF terms, the RF current would be measured by determining the magnetic field that surrounds the conductor carrying the RF power. The RF voltage would be measured by a resistive or capacitive probe positioned on or near the power conductor.
In the processing of semiconductor wafers, reactors (such as plasma reactors) are used to process the wafers. One such use of reactors is in etching a layer(s) formed on a wafer. In employing this technique, electrical power is coupled to the reactor from an electrical source. Typically, this electrical energy has a frequency in the RF range. Generally, reactor process parameters are determined by measuring and monitoring the numerous parameters, one of which is the measuring of the RF power. RF power is determined by measuring the RF voltage (V) and the RF current (I) components of the RF power coupled to the reactor . It is generally desirable to obtain the V and I measurements as close to the reactor as possible in order to obtain a true representation of the actual RF V and I values entering the reactor. Thus, a common practice for measuring RF power (V * I, where * is the mathematical operator denoting the scalar product of the voltage and current vectors) is by installing a V and I sensor in series with the transmission medium coupling the RF power to the reactor.
By utilizing a voltage "pick-off" probe and a current "pick-off" probe, a sample of the V & I from the transmission medium (such as a transmission line or a waveguide) can be obtained. For voltage pick-off, a common practice is to use a voltage divider circuit coupled to the transmission line. For current "pick-off", a common practice is to use a coil (wire loop) positioned adjacent to a driven conductor of a transmission line, in order to obtain a sample of the magnetic flux generated around the transmission line. One such RF V and I sensor (or monitor) is described in U.S. Pat. No. 5,467,013.
As stated earlier, it is difficult to obtain accurate measurements consistently and repeatedly. One reason is associated with the calibration of the sensor and another reason is associated with some mismatch of impedance encountered between the "pick-off" device and the measuring device. Sometimes, such sensors are intrusive and can disrupt the on-going process. For example, with current sensors, the calibration of the measurement depends on the amount of flux "cutting" across the pick-off coil. Accordingly, where not all flux lines cut across the pick-off coil, the sensor must be calibrated for this leakage flux.
In reference to the mismatch of impedance between the pick-off circuit and the measuring device, it is desirable to reduce such a mismatch as small as possible. Accordingly, it is the practice to have the measuring device or instrument as close as possible in distance to the sensor. However, there typically still is some amount of mismatch associated with the pick-off circuitry. Thus, some amount of measurement error is associated with this mismatch in prior art sensors.
The present invention attempts to rely more on the first principles to remove or significantly reduce the amount of error introduced in an RF sensor.